(1) Field of the Invention
The present invention relates to a coil-annealing spacer for use in box annealing of a coil-rolled metal strip for mainly removing a working strain.
(2) Description of the Prior Art
For box annealing of a metal strip coil, there is ordinarily adopted a method in which a coil is placed on a base having a base fan arranged on a central opening thereof, another coil is placed on this coil through a coil-annealing spacer (i.e. diffuser or convector), two to about six coils as a whole are piled in the stacked state, the assembled coils are covered with an inner cover and an outer cover, the inner cover is heated from the outside for a predetermined time, the outer cover is removed to naturally cool the inner cover, the inner cover is then forcibly air-cooled from the outside, the inner cover is also removed after the coil temperature is sufficiently lowered, and the coils are taken out. Incidentally, the atmosphere within the inner cover is air-purged in advance before annealing and substituted by a reducing or non-oxidizing gas, and during annealing, the pressure within the inner cover is maintained at a level slightly higher than the atmospheric air pressure and the atmosphere gas within the inner cover is forcibly circulated by the base fan.
In this annealing process, propagation of heat, either heating or cooling, is effected by radiation and convection, and the coil-annealing spacer exerts a function of promoting propagation of heat particularly by convention (contact of the atmosphere gas with the metal).
Various coil-annealing spacers differing in the shape have been designed and practically used for improving such heat transfer function, removing the heat strain and facilitating the manufacture of the spacer. For example, there can be mentioned a coil-annealing spacer in which some volute notches intersecting the involute type ribs are formed on each of upper and lower discs so as to enhance the contact efficiency between the side face of a coil (end edge of the strip) and the atmosphere gas (see Japanese Utility Model Publication No. 55-24130), a coil-annealing spacer in which plural guide ribs forming volute passages are inserted between two plates, the periphery of one plate is expanded over the peripheral edge of the other plate and said one plate is provided with a curved annular portion bent toward the other plate to change the flowing direction of the atmosphere gas (see Japanese Patent Application Laid-Open No. 56-166337), and a coil-annealing spacer in which metal spirals as a helical spring are connected to the entire face of at least one surface of a base plate and projecting walls are appropriately formed on the base plate to promote disturbance of a flow of the atmosphere gas (see Japanese Utility Model Publication No. 49-11296).
In each of these conventional coil-annealing spacers, however, the central portion of the coil between the inner and outer diameters of the coil, that is, the central portion of the coil in the radial direction, is most difficulty heated, and the temperature of this central portion is lowest at the heating step, i.e. the central portion tends to be the lowest-temperature point. While on the other hand, since the central portion is most difficulty cooled at the cooling step, the central portion tends to be the highest-temperature point. Accordingly, the annealing time cannot be shortened enough and homogenization of the produced strip quality is limited.
This tendency is a fatal defect of box annealing, and the problem has not been solved by the conventional techniques.
Incidentally, by the term "box annealing" is meant an idea of box annealing inclusive of continuous or semicontinuous coil annealing.